diff --git a/systems/errors.py b/systems/errors.py
index d867f77..a4fc70f 100644
--- a/systems/errors.py
+++ b/systems/errors.py
@@ -47,6 +47,10 @@ def __str__(self):
         return "%s for formula '%s'" % (self.__class__.__name__, self.formula)
 
 
+class MismatchedParens(FormulaError):
+    pass
+
+
 class CircularReferences(IllegalSystemException):
     def __init__(self, cycle, graph):
         self.cycle = cycle
diff --git a/systems/lexer.py b/systems/lexer.py
index 1626bcb..80a1a28 100644
--- a/systems/lexer.py
+++ b/systems/lexer.py
@@ -67,7 +67,13 @@ def lex_formula(txt):
             prev_tokens = groups.pop()
             prev_tokens.append((TOKEN_FORMULA, tokens))
             tokens = prev_tokens
-        elif c in (WHITESPACE, NEWLINE):
+        elif c == WHITESPACE:
+            if acc:
+                tokens.append(lex_value(acc))
+            acc = ""
+        elif c == NEWLINE:
+            if groups:
+                raise systems.errors.MismatchedParens(txt)
             if acc:
                 tokens.append(lex_value(acc))
             acc = ""
diff --git a/tests/test_parse.py b/tests/test_parse.py
index e5bf94c..ef59c1a 100644
--- a/tests/test_parse.py
+++ b/tests/test_parse.py
@@ -51,7 +51,7 @@ def test_parse_complex_formula(self):
 
         names = ['Hires', 'Developers', 'Ideas', 'Projects', 'Started', 'Finished']
         for name in names:
-            self.assertEquals(name, model.get_stock(name).name)
+            self.assertEqual(name, model.get_stock(name).name)
 
         results = model.run(rounds=10)
         for row in results:
@@ -128,6 +128,16 @@ def test_parens(self):
         self.assertEqual(7.5, final['C'])
         self.assertEqual(40, final['D'])
 
+    def test_missing_parens(self):
+        "There is a missing paren in the rate definition"
+        txt = """
+        [A]
+        A > B @ Rate(1 * (1 + (1 * 1))
+        """
+        with self.assertRaises(systems.errors.MismatchedParens):
+            m = parse.parse(txt)
+            results = m.run()
+
     def test_conflicting_stock_values(self):
         txt = """
         a(10) > b @ 1